It is well known to apply filters to images to improve their characteristics.
U.S. Pat. No. 7,072,525, Covell discloses an adaptive filter for filtering a target version of a visual image that is produced by processing an original version of the visual image, the characteristics of the adaptive filter being determined in accordance with one or more characteristics of the original version of the visual image. The orientation and/or strength of filtering of the adaptive filter are adjusted based on local properties of the original image, which can enable the adaptive filter to avoid introducing blurring across true edges in the image.
U.S. Pat. No. 6,823,086, Dolazza discloses a system for adaptively filtering an image so as to reduce a noise component associated with the image. The system includes an image analyzer for determining image parameters related to the image. The system also includes a spatial filter, having an adjustable kernel responsive to the image parameters, for filtering the image sequence. The image analyzer manipulates the filter kernel as a function of the image parameters so that the system produces a filtered image, adaptable in real time, as a function of the unfiltered image, external rules, predetermined constraints, or combinations thereof. The spatial filter includes a time-invariant section and an adaptable section. The time-invariant section applies a plurality of filters to the image, each of the filters having a distinct frequency response, so as to produce a plurality of distinct filtered outputs. The adaptable section scales each of the plurality of distinct filtered outputs with a corresponding distinct weighting value to produce a plurality of scaled filtered outputs, and combines the plurality of scaled filtered outputs to produce a composite filtered output.
In Covell and Dolazza, several 2-D low pass filters, each with a distinct frequency response, are applied to the image and the outputs are weighted in order to produce a composite filtered output.
As such, the complexity of U.S. Pat. Nos. 7,072,525 and 6,823,086 is high. Also, these patents require an image analyzer or another image in order to decide on the behavior of the adaptive filters, i.e. at least one pass over the original image and the target image is necessary.
U.S. Pat. No. 6,335,990, Chen et al, discloses filtering in the spatial and temporal domain in a single step with filtering coefficients that can be varied depending upon the complexity of the video and the motion between the adjacent frames. The filter comprises: an IIR filter, a threshold unit, and a coefficient register. The IIR filter and threshold unit are coupled to receive video data. The IIR filter is also coupled to the coefficient register and the threshold unit. The IIR filter receives coefficients, a, from the coefficient register and uses them to filter the video data received. The IIR filter filters the data in the vertical, horizontal and temporal dimensions in a single step. The filtered data output by the IIR filter is sent to the threshold unit. The threshold unit compares the absolute value of the difference between the filtered data and the raw video data to a threshold value from the coefficient register, and then outputs either the raw video data or the filtered data.
Chen uses an IIR filter and a threshold unit and output the raw video data or filtered data. As such, the IIR filter operates on its previous outputs and the pixel values.
Referring to FIG. 1, US 2004/0213478, Chesnokov, discloses an image processing method comprising the step of processing an input signal to generate an adjusted output signal, wherein the intensity values I(x,y) for different positions (x,y) of an image are adjusted to generate an adjusted intensity value I′(x,y) in accordance with:
            I      out        =                            ∑                      i            =            0                    N                ⁢                                            α              i                        ⁡                          (              I              )                                ⁢          LPF          ⁢                                          ⁢                                                    Ω                i                            ⁡                              [                                                      P                    i                                    ⁡                                      (                                          F                      ⁡                                              (                        I                        )                                                              )                                                  ]                                      ·                                          Q                i                            ⁡                              (                                  F                  ⁡                                      (                    I                    )                                                  )                                                        +                        (                      1            -                          α              i                                )                ⁢        I              ,where Pi(γ) is an orthogonal basis of functions of γ defined in the range 0<γ<1; Qi(.) are anti-derivatives of Pi(.):
            Q      i        ⁡          (              F        ⁡                  (          I          )                    )        =            ∫      0              F        ⁡                  (          I          )                      ⁢                            P          i                ⁡                  (          η          )                    ⁢              ⅆ                                  ⁢        η            or an approximation thereto; LPFΩis an operator of low-pass spatial filtering; Ωi is a cut-off frequency of the low-pass filter; F(.) is a weighting function; and where 0<α<1.
The output of the weighting function F(.) is monotonically decreasing with higher values of the pixels. There is a feedback from the output of the filtered sequence and the method can receive information other than from the image. For example, an amplification factor can be added to the linear or the logarithmic multiplication block and can be computed from a preview using an integral image. As such, in Chesnokov, significant processing steps are applied to the input signal, making the method quite complex and the output image is a weighted sum of the original and the processed image.